Preview rendering the current view from the viewport?
Blender 2.8+
This is now accomplished with the fully interactive Rendered viewport shading mode.
Click the rightmost sphere icon in the top right of the 3D viewport to enter Rendered mode, which displays the scene (more or less) exactly as it will be rendered:

Alternatively, hit Z and select the Rendered mode from the Shading pie menu:

To remove any overlays, outlines, or gizmos covering up parts of the rendered view, turn off Gizmos and Overlays by clicking the icons beside the dropdown arrows left of the shading mode:


Blender 2.69 – 2.79
Use the key combination ShiftZ to switch to Rendered mode. Hit the key combination again to return to Solid mode. Alternatively, switch the Viewport Shading mode between the white sphere (Solid) and pink sphere (Rendered) in the 3D Viewport header/footer bar.

This lacks viewport overlays, however, so it will be difficult to work in fully rendered mode. Nonetheless, it is possible to pivot the view and look around.

Blender 2.68 and Earlier
Switch the Viewport Shading mode between Solid and Rendered as described in the section above. However, the ShiftZ shortcut is not bound by default in earlier versions of the software. But you can bind it yourself:
Open the User Preferences (File &gt; User Preferences...) and switch to the Input tab. From the expandable list, locate 3D View and expand it, then also expand 3D View (Global) immediately below.

Scroll to the bottom of the expanded list and you will find an Add New button.

Click Add New, then expand the new entry entitled &quot;none&quot;.

With the new, unbound keymap entry expanded:

Replace none with wm.context_toggle_enum which should lead to three new text fields appearing beneath.
Bind the Z key by clicking the key field (contains the default A) and press Z.
Check the Shift keyboard modifier checkbox.
Fill the new three values, starting with space_data.viewport_shade for Context Attribute.
Next, enter SOLID for Value 1 (beneath Context Attribute).
Finally, enter RENDERED for Value 2 (right of Context Attribute).


To save these changes for future use, click Save User Settings at the footer of the User Preferences window which will update the startup file with the currently open one that now includes the added keymap. Make sure this is a clean startup file.